Modulating retroreflectors using multiple quantum well technology are described in U.S. Pat. No. 6,154,299, the disclosure of which is incorporated by reference in its entirety. Additional information regarding multiple quantum well retroreflectors is found in G. C. Gilbreath, et. al., “Progress in development of multiple quantum well retromodulators for free-space data links”, Optical Engineering, 42(6), 1611-1617 (2003), in W. S. Rabinovich, et.al, “Free-space Optical Free-Space Communications Link at 1550 nm using Multiple Quantum Well Modulating Retroreflectors in a Marine Environment”, Optical Eng., 44(5), pp. 056001-056012 (2005).
Retroreflectors are also disclosed in Mark. L. Bierman, et. al., “Design and analysis of a diffraction limited cat's eye retroreflector”, Opt. Eng., 41(7), pp. 1655-1660 (2002), R. J. Walters et al., “Photovoltaically powered modulating retroreflectors”, Optical Engineering, March 2006, Vol. 45, Issue 3, pp. 034003-1-034003-6, D. S. Katzer et al., “Optimization of buffer layers for InGaAs/AlGaAs PIN optical modulators grown on GaAs substrates by molecular beam epitaxy”, J. Vac. Sci Technol. B, May/June 2000, Vol. 18, Issue 3, pages 1609-1613.
A hybrid free space infrared communication link that supports audio transmission is described in J. L. Murphy et al., “FM-MRR Analog Audio System”, Proceedings SPIE 5892 (September 2005).